This diff introduces adding of the getStringToSign C++ function which is exposed to the Rust using the CXX bridge to get the string to verify from the DynamoDB database by the deviceID.
To create a new session client first gets the verification string from the server and then sign it with the private key. The randomly generated string is saved into the DynamoDB table for the corresponding deviceID during the request. To verify the string in the following D5947 and Rust verify function in D5945 we should get this string.
Linear task: ENG-2492